Let’s learn rack by implementing it from scratch

8 minute read

Rack is a framework for ruby web frameworks. If you developed apps in ruby frameworks like rails, hanami, Sinatra, you already used Rack. Almost all ruby web frameworks use rack under the hood, If you are already familiar with rack then you can skip next step and go to re building part

Introduction To Rack

Rack provides minimalistic API to interact, First let’s have a look at rack

A rack app is an object which takes request environment hash and provides array of 3 elements the output, rack object should respond to the method call

  • The HTTP response code
  • Headers hash
  • response body object which responds to each

The obvious question here is why does rack says about responding to the particular method. This one of the powerful paradigm available in ruby called duck typing. ie rack doesn’t care about the object or it’s implementation as long as it responds to the particular method

Let’s take a look at a simple example.

Install rack

  gem install rack

create a file with name config.ru

# config.ru
run Proc.new { |env| ['200', {'Content-Type' => 'text/html'}, ['Hello World\'d']] }

and run the command rackup

Now open the browser and vist localhost:9292

Congratulations !, You just made a rack app with just a single line

Now let’s have look at the above example

We have a proc object which responds to call method. 200as status  code , an array consist of {'Content-Type' => 'text/html'} as Response header and ['Hello World \'d'] as body

Since rack do not care about the kind of rack object, we can do the same using class or an object


class SuperCoolApp
  def call(env)
    ['200', {'Content-Type' => 'text/html'}, ['Hello World\'d']]
  end
end
run SuperCoolApp

class CoolApp
  def self.call(env)
    ['200', {'Content-Type' => 'text/html'}, ['Hello World\'d']]
  end
end

run SuperCoolApp.new

But this doesn’t do anything interesting, this will just display Hello World for all requests . Because we were returning same output without even considering the parameter env . Let’s have a look into by returning env hash as output

class CoolApp
  def self.call(env)
    ['200', {'Content-Type' => 'text/html'}, [env.inspect]]
  end
end

Now run rackup and goto localhost:9292/hello/world Output will be something like this

{"rack.version"=>[1, 3], "rack.errors"=>#>>, "rack.multithread"=>true, "rack.multiprocess"=>false, "rack.run_once"=>false, "SCRIPT_NAME"=>"", "QUERY_STRING"=>"", "SERVER_PROTOCOL"=>"HTTP/1.1", "SERVER_SOFTWARE"=>"puma 3.6.0 Sleepy Sunday Serenity", "GATEWAY_INTERFACE"=>"CGI/1.2", "REQUEST_METHOD"=>"GET", "REQUEST_PATH"=>"/hello/world", "REQUEST_URI"=>"/hello/world", "HTTP_VERSION"=>"HTTP/1.1", "HTTP_HOST"=>"localhost:9292", "HTTP_CONNECTION"=>"keep-alive", "HTTP_UPGRADE_INSECURE_REQUESTS"=>"1", "HTTP_USER_AGENT"=>"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36", "HTTP_ACCEPT"=>"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8", "HTTP_DNT"=>"1", "HTTP_ACCEPT_ENCODING"=>"gzip, deflate, sdch, br", "HTTP_ACCEPT_LANGUAGE"=>"en-US,en;q=0.8,ml;q=0.6", "SERVER_NAME"=>"localhost", "SERVER_PORT"=>"9292", "PATH_INFO"=>"/hello/world", "REMOTE_ADDR"=>"127.0.0.1", "puma.socket"=>#, "rack.hijack?"=>true, "rack.hijack"=>#, "rack.input"=>#>, "rack.url_scheme"=>"http", "rack.after_reply"=>[], "puma.config"=>#"development", :pid=>nil, :Port=>9292, :Host=>"localhost", :AccessLog=>[], :config=>"/home/tachyons/code/rack/config.ru"}, {:log_requests=>false, :environment=>"development", :binds=>["tcp://localhost:9292"], :app=>#, @content_length=nil>>, @logger=#>>>>}, {:environment=>"development"}, {}], @defaults={:min_threads=>0, :max_threads=>16, :log_requests=>false, :debug=>false, :binds=>["tcp://0.0.0.0:9292"], :workers=>0, :daemon=>false, :mode=>:http, :worker_timeout=>60, :worker_boot_timeout=>60, :worker_shutdown_timeout=>30, :remote_address=>:socket, :tag=>"rack", :environment=>"development", :rackup=>"config.ru", :logger=>#>, :persistent_timeout=>20}>, @plugins=#>, "rack.tempfiles"=>[]}

Now change url and see the changes output

To make it clear Let’s build a simple app to hello

class CoolApp
  def self.call(env)
    ['200', {'Content-Type' => 'text/html'}, [ "Hi " + env['REQUEST_PATH'].split('/').join(" ")]]
  end
end

run CoolApp

Run rackup again, and go to localhost:9292/aboobacker/mk

App will respond “Hi aboobacker mk”

You can implement your own logic using the env variable provided by rack

Rack also provides Rack Request Abstraction which provides a convenient interface to a Rack environment.

But that is not the end, rack also provides feature called middleware, which let you use multiple rack apps as pipeline . ie output of one rack app will feed as input to next rack app . Let’s check that by one example

class ReverseOutput
  def initialize(app)
    @app = app 
  end

  def call(env) 
    status, headers, body = @app.call(env) 
    body = body.map { |msg| msg.reverse } 
    [status, headers, body] 
  end 
end 

class CoolApp
  def self.call(env)
    ['200', {'Content-Type' => 'text/html'}, [ "Hi " + env['REQUEST_PATH'].split('/').join(" ")]]
  end
end

use ReverseOutput
run CoolApp

Here we made a simple middleware ReverseOutput which will reverse the response body, You can add any number of middlewares like this, also you can use pre defined middlewares provided by the rack and open source general purpose middlewares. List of middlewares

Building from scratch

Now let’s have a look at how the rack works by making a rack like library from scratch, Let’s name it Srack. But one obvious question here is why rackup file is config.ru, not config.rb ? . Also from where the methods like use, run etc are coming

Let’s look at our first code sample in a different way

# app.rb
  Rack::Builder.app do
    run Proc.new { |env| ['200', {'Content-Type' => 'text/html'}, ['Hello World\'d']] }
  end

Here we can see that config.ru is a block that is to be passed to Rack::Builder.app method

bundle gem srack

Now remove all TODOs from srack.gemspec So that we can run the test cases . Now if we run test cases it will show one failure message

Failed examples:

rspec ./spec/srack_spec.rb:8 # Srack does something useful

And it is true, we haven’t done anything useful yet

First thing we have to do is to build an executable equiallant to rackup, Let’s call it srackup

touch exe/srackup
#!/usr/bin/env ruby

require "srack"
Srack::Server.start

I copied above file from rack repo to make sure that we are following the same way . Since we haven’t implemenetd Srack::Server this won’t work yet . So let’s make that first

Since we have to make instance of Rack::Server we can make it as a class and define start as the class method

module Srack
  class Server
    def self.start
    end
  end
end

Now we have Srack::Server.start method. But it is doing nothing. Since we want Server object, we can delegate our start method to it’s instance method.

module Srack
  class Server
    def self.start
      new.start
    end

    def start
    end
  end
end

Now let’s set some default options for our app

module Srack
  class Server
    def initialize
     @options = default_options
    end

    def self.start
      new.start
    end

    def start
    end

    private
    
    def default_options
      {
        environment: "localhost",
        Port: "9393",
        Host: "localhost"
      }
    end
  end
end

Now we have to build the app from config.ru or the file specified as argument, we can store it in @options hash with the key config

module Srack
  class Server
    def initialize
      @options = default_options 
      @options[:config] = ARGV[0] if ARGV[0]
      @app = build_app
    end

    def self.start
      new.start
    end

    def start
    end

    private
    
    def default_options
      {
        environment: "localhost",
        Port: "9393",
        Host: "localhost",
        config: 'config.ru'
      }
    end

    def build_app
      Builder.parse_file(@options[:config])
    end
  end
end

Here we are we are using the Srack::Builder to parse the config file and load app from it . Let’s implement that logic in Builder factory

module Srack
  class Builder
    def self.parse_file(config)
      config_file = ::File.read(config)
      new_from_string(config_file)
    end

    def self.new_from_string(builder_script)
      eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app"
    end
  end
end

The first method is self-explanatory, it just read the file and passes the file body to new_from_string . The method new_from_string takes the file contents, convert it into a proc and pass to Rack::Builder.new . So that we can execute the contents of config.ru in the context of the builder

Remember our first rack app ?

# config.ru
run Proc.new { |env| ['200', {'Content-Type' => 'text/html'}, ['Hello World\'d']] }

In order to execute this

  1. Builder class should accept block for initialize method
  2. And execute it within the context of Builder object
  3. Builder class also should have methods run and to_app as setter and getter Let’s see it in code
    module Srack
      class Builder
     def initialize(&block)
       instance_eval(&block) if block_given?
     end
    
     def run(app)
       @app = app
     end
    
     def to_app
       @app
     end
    
     def self.parse_file(config)
       config_file = ::File.read(config)
       new_from_string(config_file)
     end
    
     def self.new_from_string(builder_script)
       eval "Srack::Builder.new {\n" + builder_script + "\n}.to_app"
     end
      end
    end
    

Now we have Builder class, But start method in Srack::Server class is still empty, In order to do that we have to connect to some real server. Remember when we mentioned rack is an interface to web servers?

module Srack
  class Server
    ...
    def start
      server.run @app, @options
    end

    private

    def server
      @server ||= Srack::Handler.default
    end
    ...
  end
end

Srack will have handlers for each type of servers, So that we can global api for handlers, ie all handlers should respond to run method with 2 arguments @app and @options

# lib/srack/handler.rb`
module Srack
  module Handler
    autoload :Thin, 'srack/handler/thin'
    def self.default
      Handler::Thin
    end
  end
end
# lib/srack/handler/thin.rb
require 'thin'
module Srack
  module Handler
    class Thin
      def self.run(app, options = {})
        host = options[:Host]
        port = options[:Port]
        args = [host, port, app, options]
        server = ::Thin::Server.new(*args)
        server.start
      end
    end
  end
end

Here made handler module which can accomodate multple handlers, In this example we used thin as the default server . To use thin inside our app, we have to include it in our srack.gemspec

spec.add_dependency "thin"

Now you can build the gem to test

gem build srack.gemspec
gem install srack-0.1.0.gem

Now our srack is capable for running our first rack app Just goto the directory with the file config.ru and run srackup

Implementing middleware

As discussed earlier one of the widely used feature in the rack is middleware. Let’s see how it works

A middleware will take the output(triplet) of rack app and modify it and give to next middleware or the app

We can make some tweaks in Srackup::Builder to accommodate this

...
class Builder
  ...
  def initialize(&block)
      @use = []
      instance_eval(&block) if block_given?
    end

    def run(app)
      @run = app
    end

    def use(middleware, *args, &block)
      @use << proc {|app| middleware.new(app, *args, &block)}
    end

    def to_app
      app = @run
      app = @use.reverse.inject(app) { |a,e| e[a] }
      app
    end
    ...
end
...

Here we defined an extra method use which will accept middleware as input. Also we have a new instance variable array @use which will store procs which accept app as input and returns new middleware object in return

Also, we changed to_app in such way that middlewares will be executed in the reverse order of calling

Now our app can also handle middlewares

If something is missing, or getting some errors, you can cross check with my repo here

References

  • https://github.com/rack
  • http://www.kavinder.com/blog/2014-10-10-rebuild-a-gem-rack/
  • http://www.kavinder.com/blog/2014-10-10-rebuild-a-gem-rack/

Updated:

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